Projection optical system and projection exposure apparatus having the same

ABSTRACT

A projection optical system includes a plurality of lenses that cause birefringence and at least one optical element for substantially eliminating the birefringence caused by the plurality of lenses. The at least one optical element is detachably mounted on the projection optical system.

[0001] This application is a divisional application of copending U.S.patent application Ser. No. 09/423,735, filed Mar. 13, 2000, which is acontinuation-in-part application of U.S. patent application Ser. No.09/123,443, filed Jul. 28, 1998, now abandoned.

FIELD OF THE INVENTION AND RELATED ART

[0002] This invention relates to a projection optical system and aprojection exposure apparatus having the same, for the manufacture ofdevices such as semiconductor devices, CCDS, or liquid crystal devices,for example. In another aspect, the invention is concerned with a devicemanufacturing method using such a projection exposure apparatus. Thepresent invention is particularly suitably usable in a projectionexposure apparatus of a step-and-repeat type or step-and-scan type.

[0003] The density of semiconductor devices such as a DRAM or CPU, forexample, has increased considerably. In the latest devices, a circuitpattern of a size not greater than 0.25 micron is required. Projectionexposure apparatuses, called a stepper, are widely used because of theirability of forming such a fine pattern precisely. In such steppers, apattern of a reticle is illuminated with light of a short wavelength inthe ultraviolet region, and it is projected through a projection opticalsystem onto a semiconductor (silicon) wafer in a reduced scale, wherebya fine circuit pattern is formed on the wafer.

[0004] For precision transfer of a reticle pattern, many strictconditions are applied to the projection optical system. Since thepattern size being resolvable with the projection optical system is inan inverse proportion to the numerical aperture (NA), the designingshould be made to enable enlargement of the numerical aperture.Additionally, the aberration must be corrected precisely over the wholeregion corresponding to the semiconductor chip.

[0005] The designing can be done with the aid of high-speed computersand designing software. Naturally, for production of a projectionoptical system, it is necessary to make every lens of the projectionoptical system very precisely, exactly in accordance with the design.But, in addition to this, much attention has to be paid to the glassmaterial or materials to be used. Since the refractive index of a glassmaterial has a large influence to the imaging characteristic of aprojection optical system, the uniformity thereof is very strictlycontrolled, generally to an order of 10⁻⁶ or less. Further, thebirefringence or double refraction property of a glass material islargely influential to the imaging characteristic and, therefore, themagnitude thereof should be suppressed to about 2 nm/cm, as is known inthe art.

[0006] However, with a glass material for a projection optical systemwhich may have a largest diameter of 200 mm, it is very difficult tocontrol the double refraction property so precisely, uniformly over thewhole surface. Usually, for reasons to be described below, birefringencewould be produced to some degree.

[0007] A first reason is attributable to the manufacturing process of aglass material. For light in the ultraviolet region, currently, a quartz(silica) glass is widely used. Thus, the following explanation will bemade with reference to quartz glass. As compared with the opticalcrystals, quartz glasses to be used as a lens glass material has nodirectionality in its structure. Therefore, in an idealistic state, nobirefringence is produced.

[0008] However, in quartz glasses, birefringence which might beconsidered as being attributable to remaining stresses such as thermalhysteresis or impurities may be observed experimentally. While themanufacture of quartz glass may be based on a direct method, a VAD(vapor axial deposition) method, a sol-gel method, or a plasma burnermethod, for example, in any of these methods, it is difficult forcurrent technology to reduce a mixture of impurities to a level that canbe disregarded.

[0009] Further, in cooling a quartz glass being formed in a hightemperature state, it may be possible to reduce the stress, resultingfrom differences in the way of being cooled between the surface portionand the inside portion of the glass material (i.e., the stress due tothermal hysteresis), to some extent by a thermal treatment such asannealing, for example. But, in principle, it is difficult to completelyremove it.

[0010] Referring now to FIG. 24, the process of manufacturing a lenselement to be used in a lithographic projection optical system will bedescribed. First, an ingot 100 of quartz glass is produced with arevolutionally symmetric shape. It is then sliced with a requiredthickness, by with a disk-like member 101 is provided. Since the ingot100 is produced constantly symmetrically with respect to its centralaxis 100 a, distribution of impurities remaining in the member 101 ordistribution of stresses therein due to thermal hysteresis appear, as amatter of course, symmetrically with respect to the central axis 101 a.At a final stage, cutting and polishing are made to the member 101,whereby a lens element 102 is provided.

[0011] Now, distortion, which may appear when impurities are mixed intothe spigot, will be explained. FIG. 25 is a sectional view of the ingot100. The peripheral hatching at 103 in this example shows a portion witha high impurity density. During an annealing process, the ingot 100 isheated. In the state with heat applied, the inside stress reduces tosubstantially zero. Through gradual cooling from that state,idealistically, a material without inside stress at room temperature canbe provided. However, if impurities are mixed, the thermal expansioncoefficient of the material changes. If the thermal expansioncoefficient increases with the mixture of impurities, as a matter ofcourse, it causes an increase of contraction during the cooling process.

[0012] As a result, although there is no stress in the heated state, theperipheral portion contracts largely with a temperature decrease. Ifparticular attention is paid to the central portion of the glassmaterial where a light flux is going to pass, it receives contractionfrom the peripheral portion as depicted by arrows in FIG. 25. That is,inside stresses are produced. The inside stress is a cause forbirefringence.

[0013] A second reason is attributable to a change, with time, of quartzglass when used in a stepper. As is known in the art, if light from ashort-wavelength light source such as a KrF or ArF laser is projected toa quartz glass, a phenomenon called “compaction” may occur. Althoughdetails of how it occurs are not described here, what can be observed inthat phenomenon is that the refractive index of the portion throughwhich the light has passed increases but the volume of that portiondecreases.

[0014] In FIG. 26, if laser light is projected to a hatched region 111of the disk-like member 110, the volume of that portion is likely todecrease. Since the peripheral portion not irradiated with laser lightis not influenced by compaction, as the whole, the central portion islikely to contract whereas the peripheral portion is likely to actagainst the contraction.

[0015] In a balanced state, therefore, when particular attention is paidto the central portion of the glass material where light passes, itreceives tension forces from the peripheral portion as depicted byarrows in FIG. 27. Thus, inside stresses are produced. The inside stressis a cause for birefringence. The phenomenon described above may occursimilarly in a projection optical system of a stepper. Since thephenomenon of compaction is particularly notable with use of ArF laserlight, it may cause a large problem when a projection exposure apparatuswith a light source of an ArF laser is practically developed.

[0016] As described, practically, it is very difficult to completelyremove birefringence to be produced in a glass material. To thecontrary, the requirement for birefringence in a stepper projectionoptical system is becoming strict, more and more. For providing a higherperformance projection optical system, the number of lens elementsconstituting the projection optical system is increasing and, thus, thetotal glass material thickness is increasing. Therefore, even if thebirefringence per unit length is kept to the above-described quantity(about 2 nm/cm), the total birefringence quantity of the system becomeslarge. Further, recent shortening in the wavelength of an exposure lightsource functions to enlarge the influence of birefringence.

[0017] Specifically, a comparison will be made to a case with the use ofi-line light (wavelength 365 nm) and a case with the use of an ArF laserlight source (wavelength 193 nm). If, for example, the whole opticalsystem has a birefringence property of 100 nm, in the case of i-linelight of 365 nm wavelength, it corresponds to a wavefront aberration of100/365=0.27 wavelength. For an ArF laser light source of 193 nmwavelength, it corresponds to a wavefront aberration of 100/193=0.52wavelength. Thus, for the same birefringence, the influence to animaging characteristic is larger with a shorter wavelength.

[0018] As regards an optical glass material having birefringence incentral symmetry, Japanese Laid-Open Patent Application, Laid-Open No.107060/1996, shows the use of lens elements made of different glassmaterials having different birefringence quantities, and suggestsreduction of adverse influence to the imaging characteristic byoptimizing a combination of the glass materials. However, increasing arequirement to further improve the precision of a projection opticalsystem cannot be met even by such a method. It is, therefore, desirableto cancel the birefringence itself of a glass material.

SUMMARY OF THE INVENTION

[0019] It is an object of the present invention to provide an improvedprojection optical system and/or an improved projection exposureapparatus having the same, for the manufacture of devices such assemiconductor devices, CCDs, or liquid crystal devices, for example, bywhich at least one of the problems described above can be solved.

[0020] It is another object of the present invention to provide a devicemanufacturing method using such a projection exposure apparatus.

[0021] In accordance with an aspect of the present invention, there isprovided a projection optical system for projecting a pattern of a firstobject onto a second object, wherein said projection optical system isprovided with birefringence correcting means for correctingbirefringence of an optical element of said projection optical system.

[0022] Said birefringence correcting means may comprise at least oneoptical member having a predetermined form birefringence.

[0023] Said at least one optical member may be arranged so that adistribution, including a distribution of form birefringence produced bysaid at least one optical member, is effective to cancel thebirefringence to be produced by an optical element of said projectionoptical system.

[0024] Said at least one optical member may be arranged to produce formbirefringence on the basis of a diffraction grating having a periodsmaller than a wavelength used.

[0025] Said diffraction grating may be provided on the surface of theoptical element of said projection optical system.

[0026] Said birefringence correcting means may comprise at least oneoptical member having a predetermined stress distribution.

[0027] Said at least one optical member may be arranged so that adistribution, including a distribution of stresses produced by said atleast one optical member, is effective to cancel the birefringence to beproduced by an optical element of said projection optical system.

[0028] In accordance with another aspect of the present invention, thereis provided a projection exposure apparatus, comprising: an illuminationsystem for illuminating a first object with light; and a projectionoptical system as recited above, for projecting a pattern of the firstobject illuminated with light from said illumination system, onto asecond object for exposure of the same.

[0029] In accordance with a further aspect of the present invention,there is provided a projection exposure apparatus, comprising:illuminating means for illuminating a first object with slit-like light;scanning means; and a projection optical system as recited above, forprojecting a pattern of the first object onto a second object while thefirst and second objects are simultaneously scanned in a widthwisedirection of the slit-like light, at a speed ratio corresponding to aprojection magnification of said projection optical system.

[0030] In accordance with a yet further aspect of the present invention,there is provided a device manufacturing method including a process forprinting a device pattern on a substrate by use of a projection exposureapparatus as recited above.

[0031] These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a schematic and sectional view of a projection opticalsystem according to a first embodiment of the present invention.

[0033]FIG. 2 is a schematic view for explaining the influence ofresidual distortion of an optical element.

[0034]FIG. 3 is a chart for explaining the influence of residualdistortion of an optical element.

[0035]FIG. 4 is a chart for explaining the influence of birefringence ofan optical element.

[0036]FIG. 5 is a schematic view for explaining the influence ofbirefringence of an optical element.

[0037]FIG. 6 is a graph for explaining a phase difference produced bybirefringence of a glass material according to the present invention.

[0038]FIG. 7 is a schematic view for explaining pupil coordinates of anoptical system.

[0039]FIG. 8 is a schematic view for explaining form birefringence ofbirefringence correcting means according to an embodiment of the presentinvention.

[0040]FIG. 9 is a graph for explaining a difference in refractive indexdue to directionality of polarized light.

[0041]FIG. 10 is a schematic view for explaining a phase change due tobirefringence, in the present invention.

[0042]FIG. 11 is a graph for explaining the quantity of phase change tobe produced by a birefringence correcting member.

[0043]FIG. 12 is a schematic view of a birefringence correcting memberwhich is based on form birefringence according to the present invention.

[0044]FIG. 13 is a schematic view of another birefringence correctingmember which is based on form birefringence according to the presentinvention.

[0045]FIG. 14 is a schematic view for explaining a grating depthdistribution of a fine diffraction grating according to the presentinvention.

[0046]FIGS. 15A and 15B are schematic views, respectively, of examplesof fine diffraction gratings according to the present invention, eachbeing provided on a convex surface.

[0047]FIG. 16 is a schematic view of a main portion of a stepperaccording to another embodiment of the present invention.

[0048]FIG. 17 is a schematic view of a main portion of a step-and-scantype projection exposure apparatus according to a further embodiment ofthe present invention.

[0049]FIG. 18 is a schematic view for explaining an asymmetricdistortion distribution produced in the projection exposure apparatus ofthe FIG. 17 embodiment.

[0050]FIG. 19 is a graph for explaining an asymmetric distortiondistribution produced in the projection exposure apparatus of the FIG.17 embodiment.

[0051]FIGS. 20A through 20F are schematic views for explainingbirefringence correcting means according to another embodiment of thepresent invention.

[0052]FIG. 21 is a schematic view for explaining birefringencecorrecting means according to an embodiment of the present invention.

[0053]FIG. 22 is a flow chart of a device manufacturing procedureaccording to an embodiment of the present invention.

[0054]FIG. 23 is a flow chart for explaining details of a wafer processin the procedure of FIG. 22.

[0055]FIG. 24 is a schematic view for explaining lens elementmanufacturing processes.

[0056]FIG. 25 is a schematic view for explaining inside distortionproduced by influence of impurities.

[0057]FIG. 26 is a schematic view for explaining the phenomenon ofcompaction.

[0058]FIG. 27 is a schematic view for explaining inside distortionproduced by influence of compaction.

[0059]FIG. 28 is a schematic view of a main portion of a projectionoptical system according to a fourth embodiment of the presentinvention.

[0060]FIG. 29 is a schematic view for explaining a change inpolarization state of light in the transmission through a birefringencecorrecting member according to the present invention.

[0061]FIG. 30 is a schematic view for explaining a birefringenceelliptical surface, in a positive uniaxial crystal glass material.

[0062]FIG. 31 is a graph for explaining the relationship between anincidence angle with respect to a birefringence correcting member and abirefringence correcting capacity thereof.

[0063]FIG. 32A is a graph for explaining a specific example of abirefringence correction amount.

[0064]FIG. 32B is a schematic view of a structure wherein abirefringence correcting member is integrally provided on a transparentsubstrate.

[0065]FIG. 33 is a schematic view for explaining birefringencecorrecting capacity adjusting means for a birefringence correctingmember.

[0066]FIG. 34 is a schematic view of a main portion of a projectionexposure apparatus according to a fifth embodiment of the presentinvention.

[0067]FIG. 35 is a schematic view for explaining a distribution of lightrays, with respect to an off-axis object point.

[0068]FIG. 36 is a graph for explaining the influence of birefringence,with respect to an off-axis object point.

[0069]FIG. 37 is a graph for explaining the effect of a secondbirefringence correcting member.

[0070]FIG. 38 is a schematic view for explaining incidence angles oflight rays in a case where a second birefringence correcting member isplaced in a convergent light flux.

[0071]FIG. 39 is a graph for explaining the effect of a secondbirefringence correcting member.

[0072]FIG. 40 is a schematic view of a main portion of a stepper(step-and-repeat exposure apparatus) with a projection optical system,according to a sixth embodiment of the present invention.

[0073]FIG. 41 is a schematic view of a main portion of a scanner(step-and-scan exposure apparatus) with a projection optical system,according to a seventh embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0074]FIG. 1 is a schematic view of a main portion of a projectionoptical system according to an embodiment of the present invention. Thisembodiment is applicable to a step-and-repeat type or step-and-scan typeprojection exposure apparatus. Denoted in the drawing at PL is aprojection optical system which is usually provided by several tens ofoptical elements having their aberration corrected precisely. Forsimplicity of illustration, only lens elements 1-5 are illustrated.

[0075] The lens elements 1-5 are provided by cutting and polishingquartz (silica) glass, although the glass material is not limited toquartz. Details of the structure of the projection optical system PL, asthe same is incorporated into a stepper, will be described later.Denoted at 6 is a reticle, and denoted at 7 is a wafer. The projectionoptical system PL projects a pattern on the reticle 6 surface onto thesurface of the wafer 7, in a reduced scale and through a step-and-repeator step-and-scan procedure.

[0076] Denoted in the drawing at 8 is a birefringence correcting memberaccording to the present invention. Details of the function of it willbe described later.

[0077] For quartz glass, which is the material of the lens elements 1-5,as described hereinbefore, it is difficult to completely avoidproduction of distortion, symmetrical with respect to a central axis,during the manufacturing procedure. Also, when it is used with a shortwavelength light source such as an ArF laser, there occurs distortiondue to the influence of volume contraction caused by compaction. Theinfluence of such distortion in a glass material will now be explained.Referring to FIG. 2, the stress to be produced by distortion will bedescribed first.

[0078] In FIG. 2, denoted at 10 is a circular or disk-like plate havingbeen separated, by cutting with a predetermined thickness h, from aquartz glass ingot. The illustrated plate is a glass material at apre-stage before it is formed into a lens element. Denoted at 11 is thecentral axis of the disk plate 10. Coordinates of x, y and z axes aredefined as denoted at 12. In this example, the stress along the centralaxis 11 (in the z direction) can be disregarded. Therefore, particularattention may be paid only the stress σ_(r) in the radial direction andstress σ_(θ) in the circumferential direction at a point P (FIG. 3),which is represented by coordinates (r, θ) along the x-y plane. If abalance of force in the radial direction at a small region 13 (withhatching) adjacent to point P is considered, a relation:

−σ_(r) ·rdθ+(σ_(r) +dσ _(r))(r+dr)dθ−σ _(θ) dθ·dr=0

[0079] is obtained. When simplified with high order small valuesomitted, the resultant is:

σ_(θ)−σ_(r) =r(dσ _(r) /dr)

[0080] Since distortion remaining in the glass material 10 changes withthe radial direction, generally differentiation of stress σ_(r) with aradius r does not become equal to zero. Therefor, r≠0 (other than the onthe central axis), the right hand side of equation (1) has a finitevalue other than zero. It means that, except for on the central axis 11,the stress σ_(r) in the radial direction and the stress σ_(θ) in thecircumferential direction have different values.

[0081] The influence which can be optically observed in such a case willbe explained with reference to FIGS. 4 and 5. It is assumed, as shown inFIG. 4, that light of a wavelength λ and being rectilinearly polarizedis incident on a point P (r, θ) position. Here, the direction ofpolarization of the rectilinearly polarized light is illustrated with anarrow 14. When this light passes through the disk plate 10, it isinfluenced by different refractive indexes, being different for apolarization component 15 in the radiation direction and a polarizationcomponent 16 in the circumferential direction. As a result of it, afterpassing the disk plate 10, there occurs a phase difference φ betweenthese two polarization components, as illustrated in FIG. 5. As regardsthe state of polarization of light, the rectilinear polarization isconverted into elliptical polarization. Here, by using stresses σ_(r)and σ_(θ), the phase difference φ can be expressed as follows:

φ(r)=(2π/λ)C·h{σ _(θ)(r)−σ_(r)(r)}  (2)

[0082] where C is called an “optical elastic constant,” which is a valueinherent to the material. As the light passes through the disk-likeglass material 10 in this manner, the state of polarization thereofchanges. Although the influence in each glass lens element is verysmall, as the light passes through several tens of elements, an adverseinfluence which cannot be disregarded is applied to the imagingcharacteristic of the projection optical system. In a practicalprojection optical system, the influence of equation (2) may beconsidered with respect to each element and, for the whole projectionoptical system, the quantity of phase change due to birefringence may beadded. Then, the result may be such as shown in FIG. 6.

[0083] Here, the axis of abscissa is represented by pupil coordinates ρof the optical system, in place of the radius r of the optical element.The pupil coordinates will be explained with reference to FIG. 7.Denoted in the drawing at 17 and 18 are lens elements. When particularattention is paid to a light ray 19, which passes the lens elements 17and 18, in order to designate its position, it is necessary to useplural parameters such as radius r₁ measured from the central axis(optical axis) 20 for the lens element 17 and a radius r₂ measured fromthe central axis 20 for the lens element 18. This is inconvenient.

[0084] In consideration of it, particular attention may be paid to thepupil position 21 of the whole optical system, and the light ray 19 maybe designated with the height pupil coordinates ρ where the light ray 19passes. On that occasion, the characteristic of the optical system canbe represented by a single parameter. Thus, by using ρ as the pupilcoordinates, the position of light passing through the optical systemcan be designated. The maximum value is ρ₀.

[0085] The result shown in FIG. 6 is one that can be measuredexperimentally by using an actually assembled optical system and byusing measures such as a phase modulation method, for example. However,if the precision may be sacrificed to some extent, it can be calculatedby simulation. The phase modulation method is discussed in detail by E.Mochida in “Optical Technique Contact”, Vol. 27, No. 3 (1989), forexample, and a description thereof will be omitted here. The phasemodulation method shows high sensitivity and it enables measurement at aprecision 10⁻⁸ with a value of a refractive index difference Δn due tobirefringence. Also, it has an advantage that the fast axis and slowaxis can be determined simultaneously.

[0086] Once the fast axis and slow axis are determined, the sign of φ(r) in FIG. 6 can be determined directly. Anyway, what is suggested inFIG. 6 is that, for the light which passes the center (ρ=0) of theoptical system, the quantity of phase shift due to birefringence iszero, whereas, for the light which passes the peripheral portion (ρ=ρ₀),the quantity of phase shift due to birefringence becomes up to π/4.

[0087] While a detailed explanation of the influence to the imagingcharacteristic using theoretical equations will be omitted here, whatcan be observed is the phenomenon that astigmatism of a magnitude ofabout λ/4 appears in the optical system. The aberration which can beadmitted for a projection optical system of a stepper is of a λ/100order, and a large aberration such as above would not be admitted.

[0088] In the present invention, in consideration of the above, abirefringence correcting member 8 (FIG. 1) is used in the opticalsystem, so as to cancel the phase change such as shown in FIG. 6. Inorder to cancel birefringence, which is produced symmetrically withrespect to the optical axis, as will be readily understood, a memberhaving birefringence of opposite signs symmetrical with respect to theoptical axis may be used. However, the magnitude of birefringence shouldbe substantially the same as the magnitude of birefringence to beproduced by the whole projection optical system PL.

[0089] Here, a detailed structure of the birefringence correcting member8 will be explained. As regards the material of the birefringencecorrecting member, in consideration of the need that it is transparentto exposure light and it has sufficient durability, it should be of thesame material as the optical glass used for the lens elements 1-5. Inorder that birefringence with a predetermined distribution is producedby such optical glass, in this embodiment, the phenomenon called “formbirefringence” is used.

[0090] Now, referring to FIG. 8, the form birefringence will beexplained. Denoted in the drawing at 25 is a phase type diffractiongrating which is formed on the surface of an optical glass. As bestshown with the enlarged illustration at a right hand portion of FIG. 8,the diffraction grating 25 has a period b and a depth d. The width ofthe optical glass portion that defines a fine grating is a. Here, forsubsequent discussion, a duty ratio t is defined as t=a/b. Denoted at 26is input light (wavelength=λ), being incident on the diffraction grating25. Denoted at 27 is output light which exits from the diffractiongrating.

[0091] Denoted at 28 is a polarization component, of the input light 26,in a direction parallel to the grooves of the diffraction grating 25,and denoted at 29 is a polarization component thereof in a directionperpendicular to the grooves of the diffraction grating 25. Similarly,denoted at 30 is a polarization component, of output light 27, in adirection parallel to the grooves of the diffraction grating 25, anddenoted at 31 is a polarization component thereof in a directionperpendicular to the grooves of the diffraction grating 25.

[0092] Here, as regards the period b of the diffraction grating, acondition “b not greater than λ” should be satisfied to preventproduction of diffracting light other than zero-th order diffractivelight, as the output light 27.

[0093] In FIG. 8, while in the input light 26, there is no phasedifference between the polarization components 28 and 29, with thepassage through the diffraction grating 25, there occurs a phasedifference φ between the polarization components 30 and 31. Thus, it isseen that, if the input light 26 is rectilinearly polarized light, theoutput light 27 is converted into elliptically polarized light. Such aphenomenon is called “form birefringence”, as is well known in the fieldof optics.

[0094] Details of form birefringence are described by M. Born and E.Wolf in “Principles of Optics”, 1st ed., Pergamon Press, New York, 1959,pages 705-708, or by Aoyama, et al., in “Optics”, Vol. 21, No. 5, pages369-274, 1992, for example.

[0095] A fine diffraction grating such as at 25 in FIG. 8 showsdifferent refractive indexes, being different with the direction ofpolarization of the input light 26. The refractive index n_(II) wherethe polarization of input light 26 is parallel to the grooves of thediffraction grating 25 as well as the refractive index n_(⊥) where thepolarization of input light 26 is perpendicular to the grooves of thediffraction grating 25, are expressed as follows:

n _(II) ={square root}{square root over (tn₁ ²+(1−t)n₂ ²)}  (3)

n _(⊥)=1/{square root}{square root over ((t/n ₁ ²)+[(1−t)/n ₂ ²])}  (4)

[0096] where t is the duty ratio having been defined above, n₁ is therefractive index of the member which constitutes the diffraction grating25, and n₂ is the refractive index of the medium at the side where thelight is incident. FIG. 9 shows the result of a calculation of thedependency of n_(II) and n_(⊥) upon t, with the calculation being madeunder the condition that n₁=1.6 and n₂=1.0. Further, when the groovedepth of the diffraction grating 25 is d, the phase difference φ whichappears between the polarization component parallel to the diffractiongrating grooves and the polarization component perpendicular to thediffraction grating grooves is given by:

φ=(2πd/λ)(n _(II) −n _(⊥))  (5)

[0097] From equations (3)-(5), it is seen that the value of the phasedifference can be set as desired by appropriately selecting the dutyratio t and groove depth d.

[0098] The birefringence correcting member 8 may be inserted to thepupil position 21 of the projection optical system PL in FIG. 7. Thedetailed structure thereof will now be explained, with reference to FIG.10.

[0099]FIG. 10 is a schematic view for explaining the relation of phasedifference between a polarization component in the radial direction anda polarization component in the circumferential direction, as the lightbeing designated by a pupil radius ρ in the projection optical system ofFIG. 1 goes through the lens elements 1-5 and the birefringencecorrecting member 8. If, in the glass material, the birefringence whichis symmetric with the optical axis is assumed, there occurs a phasedifference between the radial polarization component of input light andthe circumferential polarization component, as has been described withreference to FIGS. 4 and 5.

[0100] In FIG. 10, denoted at 41-44 are polarization components in theradial direction. Denoted at 45-48 are polarization components in thecircumferential direction. The phase difference between them is, ofcourse, zero until the light 40 enters the lens element 1.

[0101] Here, it is assumed in this example that there is a phasedifference φ (ρ) shown in FIG. 6 being caused as a function of the pupilradius coordinates ρ and by the birefringence produced in the lenselements 1-5. Also, it is assumed that the phase difference φ (ρ) issolved in accordance with a relation φ (ρ)=φ ₁(ρ)+φ ₂(ρ), and that thephase difference φ ₁(ρ) is taken as the phase difference produced by thelens elements 1-3 while the phase difference φ ₂(ρ) is taken as thephase difference produced by the lens elements 4-5.

[0102] Further, it is assumed that in the birefringence correctingmember 8, a phase difference φ (ρ) is produced between the polarizationcomponents in the radial direction and circumferential direction as afunction of the pupil coordinates ρ. Then, as regards the light justbefore it is incident on the birefringence correcting member 8, thephase difference between the radial direction polarization component andthe circumferential direction polarization component is given by φ ₁(ρ),whereas the phase difference just after the light passes thebirefringence correcting member 8 is given by φ ₁(ρ)+φ (ρ). Further, thephase difference after passage through the lens elements 4-5 is givenby:

φ ₁(ρ)+φ ₂(ρ)+φ (ρ)=φ (ρ)+φ (ρ)  (6)

[0103] In accordance with this embodiment of the present invention, abirefringence member which can apply a phase difference function φ (ρ)with which equation (6) results in zero is inserted into the opticalpath.

[0104] Considering this in combination with FIG. 6, it is readilyunderstood that the phase difference function φ (ρ) with which equation(6) results in zero is the one shown in FIG. 11. Namely, it has anopposite sign to the phase difference φ (ρ), but has the same absolutevalue.

[0105] As has been described, by using a fine diffraction grating 25having form birefringence and by appropriately selecting its duty ratiot and groove depth d, the phase difference between the polarizationcomponent parallel to the diffraction grating groove and thepolarization component perpendicular thereof can be set as desired.

[0106] Thus, in the birefringence correcting member 8, a finediffraction grating is formed on its surface so that it is symmetricwith respect to the optical axis. On that occasion, as shown in FIG. 12,the diffraction grating 25 may be disposed concentrically with respectto the optical axis. Alternatively, the diffraction grating 25 may bedisposed radially with respect to the optical axis, as shown in FIG. 13.

[0107] For the period of the fine diffraction grating necessary forproducing form birefringence, only the condition not greater than thewavelength is required, as has been described hereinbefore. However, ifthe period is too small, the manufacture becomes difficult. Thus, in acase where the diffraction grating is disposed radially with respect tothe optical axis, the radial direction may be divided into pluralregions such as shown in FIG. 13, and in each region the periodsatisfies the above condition.

[0108] The structures shown in FIGS. 12 and 13 have a difference infunction, such as follows. As regards the light incident on point P inFIG. 12, and between polarization component 50 in the radial directionand polarization component 51 in the circumferential direction, with thepassage of light through the member 8, the phase of the polarizationcomponent 51 in the circumferential direction can be relatively delayedrelative to the phase of the polarization component 50 in the radialdirection. On the other hand, with respect to the structure of FIG. 13,the phase of a polarization component 53 in the circumferentialdirection can be relatively advanced relative to the phase of apolarization component in the radial direction. Namely, they can be usedselectively in accordance with the sign of birefringence produced by theprojection optical system. Here, in order to produce the phasedifference φ (ρ) shown in FIG. 1, the structure of FIG. 13 may be used.

[0109] As a matter of course, the amount of phase correction should bechanged with the radial direction. To this end, the duty ratio of thefine diffraction grating or its groove depth may be changed with theradial direction. However, it is very difficult in manufacture to changethe duty ratio continuously. An example of changing the depth may besuch as shown in FIG. 14. The amount of phase correction is zero on theoptical axis of an optical system, as seen also from FIG. 11. Thus, inFIG. 14, the fine diffraction grating may have a groove depth with iszero in the neighborhood of the central axis and which increases towardthe periphery.

[0110] As regards the position where the birefringence correcting member8 is to be inserted, although it is preferably near the pupil positionof the projection optical system, it is not limited to there. Further,although in this embodiment, the birefringence correction comprises aparallel flat plate, the shape is not limited to a parallel flat plate.It may have a shape with a convex surface or concave surface, like anordinary lens element.

[0111]FIG. 15A shows an example wherein a fine diffraction grating isformed on a convex surface concentrically with respect to the centralaxis. FIG. 15B shows an example wherein a fine diffraction grating isformed on a convex surface, radially with respect to the central axis.

[0112] While the present embodiment has been described with reference toexamples wherein the birefringence correcting member is provided by asingle optical element, it may comprise plural optical members with thebirefringence correction amount shared by them. On that occasion, itbecomes possible to correct larger birefringence produced in aprojection optical system or birefringence having a complexdistribution.

[0113]FIG. 16 is a schematic view of a main portion of a step-and-repeattype exposure apparatus, according to a second embodiment of the presentinvention, with a projection optical system of the present inventionincorporated therein. Denoted in the drawing at 60 is a reticle having acircuit pattern formed thereon. Denoted at 61 is a projection opticalsystem according to the present invention, and denoted at 62 is a waferonto which the circuit pattern is to be transferred. Illumination light63 from an illumination system 67 illuminates an illumination region 64on the reticle 60 surface, and the circuit pattern formed in that region64 is transferred by the projection optical system 61 onto an exposureregion 65 on the wafer 62 in a reduced scale. In a stepper, the patternof the reticle 60 is transferred at once to the wafer 62 in a reducedscale. After this, the wafer 62 is moved stepwise by a predeterminedamount, and then the exposure is performed again. This procedure isrepeated.

[0114] Denoted in the drawing at 66 is a birefringence correctingmember. In this embodiment, the birefringence correcting member 66itself can be demounted or replaced by another, so that the amount ofcorrection thereby can be changed in accordance with the amount ofbirefringence of the projection optical system 61.

[0115]FIG. 17 is a schematic view of a main portion of a step-and-scanexposure apparatus according to a third embodiment of the presentinvention, with a projection optical system of the present inventionincorporated therein. Denoted in the drawing at 70 is a reticle having acircuit pattern formed thereon. Denoted at 71 is a projection opticalsystem, and denoted at 72 is a wafer onto which the circuit pattern isto be transferred. Illumination light 73 from an illumination system 67illuminates an illumination region 74 on the reticle 70 surface, and thecircuit pattern formed in that region 74 is transferred by theprojection optical system 71 onto an exposure region 75 on the wafer 72in a reduced scale. The step-and-scan type exposure apparatus differsfrom conventional steppers in the following points.

[0116] In a step-and-repeat type exposure apparatus, a pattern of areticle 70 is transferred to a wafer 72 at once, in a reduced scale. Onthe other hand, in a step-and-scan type exposure apparatus, a circuitpattern is illuminated with a slit-like shaped illumination region 74,and the reticle 70 and the wafer 72 are scanningly moved in synchronismwith each other, by which the whole circuit pattern of the reticle istransferred to the wafer in a reduced scale.

[0117] The coordinate system is such as at 76, wherein the scandirection 77 of the reticle 70 corresponds to a negative x axisdirection, while the scan direction of the wafer 72 corresponds to apositive x axis direction. Denoted in the drawing at 79 is abirefringence correcting member which can be demountably mounted and canbe replaced by another like the embodiment of a stepper.

[0118] Since in a step-and-scan type exposure apparatus the illuminationregion 74 has a slit-like shape, the influence of compaction whenillumination is made by use of an ArF laser does not appearsymmetrically with respect to the optical axis.

[0119] Referring to FIG. 18, the light transmission region of a lenselement of the projection optical system 71 will be explained. Denotedin the drawing at 80 is a representative lens element, being illustratedon an x-y plane with coordinates at 81.

[0120] Here, in accordance with the shape of the illumination region 74,the region on the lens element 80 through which the light passes has ashape, with hatching 82, being elongated in the y-axis direction. Thus,distortion attributable to compaction is produced in accordance withthat shape. Also, the birefringence of the optical system produced as aresult of it becomes, of course, asymmetric with respect to the opticalaxis.

[0121] The quantity of phase change due to birefringence, when depictedby use of pupil coordinates similar to that of FIG. 6, is such as shownin FIG. 19. Namely, it has different distributions in the x-axisdirection and y-axis direction. Correction of such a distributedbirefringence quantity can be met by using different groove depths,being different with respect to the x-axis and y-axis directions, suchas described with reference to FIG. 14.

[0122] Next, another embodiment of a birefringence member according tothe present invention will be described. With reference to the firstembodiment of FIGS. 1-8, as regards the specific structure ofbirefringence correcting member 8, in consideration of the need that thematerial of the birefringence correcting member is transparent toexposure light and it has sufficient durability, the same material asthe optical glass for the lens elements 1-5 is used. Also, forbirefringence, the phenomenon called “form birefringence” is utilized.

[0123] As described with reference to the first embodiment, also in thisembodiment, distortion remains in the manufacture of optical glass and,due to the influence of it, birefringence is produced. However, in thisembodiment, as compared with the first embodiment, distortion ispositively applied to an optical glass to thereby produce desiredbirefringence. This embodiment differs from the first embodiment only inthis point, and the remaining structure and function are basically thesame as that of the first embodiment.

[0124] Distortion can be left in the optical glass, by preciselycontrolling the temperature during an annealing process. Usually, theannealing process is performed so as to remove distortion remaining inthe optical glass. However, in production of a birefringence correctingmember of the present invention, the annealing process is used, on thecontrary, to produce residual distortion.

[0125] The annealing process will be described with reference to FIGS.20A to 20F. Denoted in the drawing at 130 is an optical member which isgoing to be formed into a birefringence correcting member 8. The opticalmember 13 has a disk-like shape, and the position on the member isdesignated with the distance r from the central axis 131.

[0126]FIG. 20A illustrates the state before heating for the annealingprocess. FIG. 20B shows the state in which heat is uniformly applied tothe whole optical member 130. In this state, there is substantially nostress distribution inside the optical member 130. During a coolingprocess shown in FIGS. 20C and 20D, a large stress distribution can beproduced. In FIG. 20C, gas flows to the central portion for quickcooling thereof. As a result of this, when the optical member 130returns to room temperature, there is a large residual stress producedinside. FIG. 20E shows such a stress distribution.

[0127] The stress σ_(r) in the radial direction and stress σ_(θ) in thecircumferential direction are depicted as a function of the radius 4.They coincide with each other at the center. There is a differencebetween them which increases with enlargement of radius r. If an objecthaving such a stress difference is inserted into the optical path, aphase different as can be calculated by equation (2) is produced.

[0128] Here, the stress distribution which should be produced in theoptical member is adjusted to a value which is effective to cancel thephase difference due to the birefringence of the whole projectionoptical system, as shown in FIG. 6. To this end, the temperature of gasto be blown, the blowing position and the temperature after gas blowingshould be controlled exactly. Optimum conditions therefor can bedetermined on the basis of experiments. As an example, with quickcooling by blowing air to the peripheral portion such as illustrated inFIG. 20D, a stress distribution such as shown in FIG. 20F is obtainable.

[0129] For producing residual stress in birefringence correcting member8, the temperature distribution control for an annealing process may bereplaced by changing the impurity density of the optical member 130 inthe radial direction. Similar advantageous effects are attainable withthe latter method. Further, a dynamic pressure may be externally appliedto a disk-like glass material member or a lens element or elements, bywhich a desired inside stress distribution can be provided.

[0130] In the step-and-scan type exposure apparatus shown in FIG. 17,there may be produced birefringence in the projection optical system,which is asymmetric with respect to the optical axis. Referring to FIG.21, the manner of correcting asymmetric birefringence by using abirefringence correcting member of the present embodiment in such anexposure apparatus, will now be explained.

[0131] Like the example of FIGS. 20A to 20F, the birefringence of thecorrecting member is adjusted by controlling the temperaturedistribution during the annealing process. Reference numeral 163 in FIG.21 denotes the state in which heat is uniformly applied to the wholeduring the annealing process. Reference numeral 164 denotes the stateafter cooling, with different temperature distributions defined in the xand y directions. In this state, the residual distortion inside themember has different distributions in the x and y directions. Byinserting such a member into the projection optical system, theinfluence of birefringence produced asymmetrically with respect to theoptical axis of the optical system can be corrected.

[0132] Next, an embodiment of a device manufacturing method, which usesa projection exposure apparatus such as described above, will beexplained.

[0133]FIG. 22 is a flow chart of a procedure for the manufacture ofmicrodevices such as semiconductor chips (e.g., ICs or LSIs), liquidcrystal panels, or CCDs, for example. Step 1 is a design process fordesigning a circuit of a semiconductor device. Step 2 is a process formaking a mask on the basis of the circuit pattern design. Step 3 is aprocess for preparing a wafer by using a material such as silicon. Step4 is a wafer process, which is called a pre-process, wherein, by usingthe so prepared mask and wafer, circuits are practically formed on thewafer through lithography. Step 5, subsequent to this, is an assemblingstep, which is called a post-process, wherein the wafer having beenprocessed by step 4 is formed into semiconductor chips. This stepincludes an assembling (dicing and bonding) process and a packaging(chip sealing) process. Step 6 is an inspection step wherein anoperation check, a durability check and so on for the semiconductordevices provided by step 5, are carried out. With these processes,semiconductor devices are completed and they are shipped (step 7).

[0134]FIG. 23 is a flow chart showing details of the wafer process. Step11 is an oxidation process for oxidizing the surface of a wafer. Step 12is a CVD process for forming an insulating film on the wafer surface.Step 13 is an electrode forming process for forming electrodes upon thewafer by vapor deposition. Step 14 is an ion implanting process forimplanting ions to the wafer. Step 15 is a resist process for applying aresist (photosensitive material) to the wafer. Step 16 is an exposureprocess for printing, by exposure, the circuit pattern of the mask onthe wafer through the exposure apparatus described above. Step 17 is adeveloping process for developing the exposed wafer. Step 18 is anetching process for removing portions other than the developed resistimage. Step 19 is a resist separation process for separating the resistmaterial remaining on the wafer after being subjected to the etchingprocess. By repeating these processes, circuit patterns are superposedlyformed on the wafer.

[0135] With these processes, high density microdevices can bemanufactured.

[0136] In accordance with the embodiments of the present invention asdescribed above, a birefringence correcting member being setappropriately is provided in a projection optical system. This enablessuperior correction of the birefringence property of a projectionoptical system itself or birefringence produced during the projectionexposure process. Thus, the present invention provides a projectionoptical system, a projection exposure apparatus having the same, or adevice manufacturing method using it, by which high precision patterntransfer is assured.

[0137] Particularly, even in a case wherein birefringence is produced ina glass material which constitutes a projection optical system, theinfluence thereof can be corrected or compensated for such that highprecision pattern transfer is assured. Further, the influence ofdistortion due to compaction, which is caused by absorption of ArF laserlight, for example, by the glass material, can be corrected.

[0138]FIG. 28 is a schematic and sectional view of a main portion of aprojection optical system according to a fourth embodiment of thepresent invention. This embodiment can be applied either to astep-and-repeat system or to a step-and-scan system. Denoted in FIG. 28at PL is a projection optical system, which usually comprises a few tensof optical elements having their aberrations corrected precisely. Here,the illustration is simplified, and lens elements 101-103 represent manyoptical elements.

[0139] Each of the lens elements 101-103 can be produced by cutting andpolishing a quartz lens (fused silica). Denoted at 104 is abirefringence correcting member (correcting means). Denoted at 104 is areticle, and denoted at 106 is a wafer. In operation, a pattern formedon the reticle 105 surface is projected in a reduced scale upon thewafer 106 surface, in accordance with a step-and-repeat method orstep-and-scan method.

[0140] The projection optical system of this embodiment is provided witha birefringence correcting member 104, which comprises an opticalelement made of a uniaxial crystal having a principal axis extendingalong the optical axis direction or of an optical material having adistortion distribution equivalent to that of a uniaxial crystal. Thethickness and the surface shape as well as the birefringence property ofthe birefringence correcting member 104 are so determined to cancelbirefringence to be produced at any other lens element or elements.

[0141] Here, the fused silica constituting the lens elements 101-103 hassuch a double refraction property that the orientation of an advancementphase axis is distributed radially from the optical axis. In FIG. 28,for explanation of the influence of birefringence, three light rays 107,108 and 109 emitted from a single point on the reticle 105 areillustrated. As regards the light ray 107, particularly, thepolarization components before incidence upon the lens element 101 arespecified at 110 and 111, while the polarization components afteremission from the lens element 103 are specified at 112 and 113.Further, the polarization components after emission from the correctingmember 104 are specified at 114 and 115. Here, the polarized lights 110,112 and 114 denote the polarization components being parallel to thesheet of the drawing, while the polarized lights 111, 113 and 115 denotepolarization components being perpendicular to the sheet of the drawing.

[0142] As shown in FIG. 28, the polarization components 110 and 111before entering the lens element 101 have the same wavefront. However,as it passes through the three lens components 101, 102 and 103, thereoccurs a shift (deviation) of wavefront between the polarizationcomponents 112 and 113. In the lens elements 101-103, the direction ofvibration of electric field of the polarization component 112 iscoincident with the advancement phase axis direction and, therefore, thewavefront of the polarization component 112 is relatively advanced ascompared with that of the polarization component 113. If the lightreaches the image plane (wafer surface) 106 while keeping this state,the imaging performance is degraded. In consideration of it, in thisembodiment, the birefringence correcting member 104 is provided at theposition inside the projection lens PL, which is closest to the imageplane side, to correct any shift of wavefront produced between thepolarization components 112 and 113. Thus, the light rays aretransformed into two polarization components 114 and 115 having theirwavefronts registered with each other, and they are projected upon thewafer 106 surface.

[0143] Referring to FIGS. 29 and 30, the optical function of thecorrecting member 104 will be described in detail. The correcting member104 comprises a parallel flat plate, being made of a uniaxial crystaland having a thickness d. It is placed so that the crystal axis thereof(in this case, it corresponds to a principal axis and an optical axis)is registered with the optical axis PLa direction of the projectionlens. As regards such a uniaxial crystal material having a hightransmission factor in an ultraviolet region and having a good physicaldurability, magnesium fluoride (MgF₂) may be used, for example. In thedrawings, a light ray 120 is illustrated as being perpendicularlyincident on the correcting member 104, while a light ray 121 isillustrated as being incident on the correcting member 104 with an angleθ.

[0144] As regards the light ray 120, if the polarization componentsbefore incidence are denoted at 122 and 123, these two polarizationcomponents 122 and 123 enter the correcting member 104 as an ordinaryray (field vibration direction being perpendicular to the principalaxis). As a result, there occurs no wavefront shift between thepolarization components 124 and 125 after being transmittedtherethrough.

[0145] On the other hand, when the polarization components of the lightray 121 before incidence are denoted at 126 and 127, the polarizationcomponent 127 passes through the correcting member 104 as an ordinaryray whereas the polarization component 126 passes through the correctingmember 104 as an extraordinary ray. Since the refractive index of thematerial of the correcting member 104 differs between an ordinary rayand an extraordinary ray, there occurs a deviation φ (θ) of wavefrontbetween two polarization components 128 and 129, as illustrated.

[0146] Magnesium fluoride (MgF₂), for example, is a positive crystal,and the refractive index n_(e) for an extraordinary ray becomes largerthan the refractive index n_(o) for an ordinary ray. Therefore, thewavefront of the polarization component 128 is relatively retarded ascompared with that of the polarization component 129. FIG. 30illustrates a refractive index ellipsoid wherein the refractive index ofthe uniaxial crystal is depicted on an X-Z plane. The refractive indexof a light ray having an angle θ′ with respect to the principal axis isgiven by a point A, in the case of an ordinary ray, whereas it is givenby a point B in the case of an extraordinary ray. As for the refractiveindex of the extraordinary ray, a value where θ′=90 degrees is taken asn_(e). For a usual angle θ′, it is taken as ρ(θ). It is clearly seenfrom the drawing that the refractive index with respect to a light raychanges with the direction θ′ of the light ray inside the crystal. Here,the following relation can be derived. $\begin{matrix}{{\rho \left( \theta^{\prime} \right)} = \frac{1}{\sqrt{\frac{\sin^{2}\theta^{\prime}}{n_{e}^{2}} + \frac{\cos^{2}\theta^{\prime}}{n_{o}^{2}}}}} & (7)\end{matrix}$

[0147] If geometro-optical separation of light rays due to birefringenceis disregarded, the incidence angle θ and the direction θ′ inside thecrystal are interrelated with each other in accordance with Snell's law,i.e., sin θ=n sin θ′ where n=(n_(o)+n_(e))/2. Thus, the wavefront shiftφ (θ) can be expressed as:

φ(θ′)={[ρ(θ′)−n _(o) ]d}/cos θ′  (8)

[0148]FIG. 31 illustrates this, in terms of a function of angle θ.

[0149] From the results described above, it is seen that the amount ofwavefront shift between polarization components to be produced by thebirefringence changes in accordance with the incidence angle of thelight ray. When a positive uniaxial crystal such as shown in FIG. 29 isused, the wavefront of the polarization component parallel to the sheetof the drawing can be retarded as compared with the polarizationcomponent 127, which is perpendicular to the sheet of the drawing.Therefore, any shift of wavefront produced between the polarizationcomponents 112 and 113 in FIG. 28 can be corrected and when they passthrough the correcting member 104, as depicted by polarizationcomponents 114 and 115. While the above-described description concernsthe light ray 107, a similar explanation applies also to the light ray109.

[0150] As regards the light ray 108, it is not at all influenced, interms of birefringence, by the lens elements 101-103 and the correctingmember 104. Therefore, constantly it is directed from the object plane105 to the image plane 106 in an idealistic state. If the influence ofthe birefringence produced at the lens elements 101-103 is representedby φ (θ), the system can be set, with respect to the birefringencecorrection amount φ (θ) of the birefringence correcting member 104, asbeing represented by FIG. 31, so as to satisfy a relation of φ (θ)−φ(θ)=0.

[0151] In the present embodiment, as described above, the influence ofbirefringence of the glass material is corrected by means of thecorrecting member 104, by which potential imaging characteristics of theprojection lens are realized.

[0152] In order to cancel any double refraction to be produced as aresult of the influence of a glass material of a projection lens PL, itis necessary to control the amount of wavefront shift to be produced bythe correcting member 104. To this end, the value of the difference“n_(e)−n_(o)” in refractive index between ordinary and extraordinaryrays or, alternatively, the thickness d may be changed. However, wherethe correcting member 104 is made of MgF₂, for example, the value of therefractive index difference n_(e)−n_(o) is determined definitely.Therefore, the parameter d for plate thickness is the only parameter tobe controlled.

[0153] Here, determination of the plate thickness d will be described,with reference to a case where MgF₂ is used for the material of thecorrecting member 104.

[0154] According to the data discussed in “HANDBOOK OF OPTICS II, SecondEdition” (ISBN 0-07-047974-7), Chapter 33 (page 33.64), it is suggestedthat the wavelength dependency of the refractive index of MgF₂, withrespect to an ordinary ray and an extraordinary ray, is as follows (theunit of the wavelength λ is μm): $\begin{matrix}{n_{o} = \sqrt{1 + \frac{2.31204\lambda^{2}}{{- 566.136} + \lambda^{2}} + \frac{0.39875\lambda^{2}}{{- 0.00895189} + \lambda^{2}} + \frac{0.487551\lambda^{2}}{{- 0.00188218} + \lambda^{2}}}} \\{n_{e} = \sqrt{1 + \frac{2.49049\lambda^{2}}{{- 163.124} + \lambda^{2}} + \frac{0.504975\lambda^{2}}{{- 0.00823767} + \lambda^{2}} + \frac{0.41344\lambda^{2}}{{- 0.00135738} + \lambda^{2}}}}\end{matrix}$

[0155] If a case wherein the numerical aperture NA at the image side isNA=0.7 is considered, the largest value for the angle θ will be, becauseof sin θ=0.7, approximately equal to 45 degrees. Assuming d−1 mm andcalculating a change of φ (θ) within the range of 0≦θ≦45 (degrees), weobtain the relationship as illustrated in FIG. 32A. Thus, when athickness of 1 mm is chosen for the correcting member 104, in relationto light rays (such as light rays 107 and 109 in FIG. 28) passingthrough a peripheral portion of the projection lens PL and impinging onthe image plane with an angle 45 degrees, correction of a wavefrontdeviation, due to birefringence, of a magnitude 0.0037 mm can beaccomplished. Here, it will be readily understood from the equationabove that the thickness d and the birefringence correction amount φ (θ)are in a proportional relation with each other.

[0156] As regards the wavefront deviation due to birefringence asproduced at the glass material of the projection lens PL, other than thecorrecting member 104, qualitatively it can be expressed by a function φ(θ)(θ is the angle of light ray). Specifically, if the amount ofbirefringence of the glass material at the position where the light rays107 and 109 in FIG. 28 pass through is 2 nm/cm and the total lengththrough which the light rays 107 and 109 pass the glass material is 50cm, then the amount of wavefront deviation between the polarizationcomponents 112 and 113 will be about 100 nm. When the correcting member104 has a thickness of 1 mm, wavefront deviation correction due tobirefringence of an amount 0.0037 mm can be accomplished. Thus, forcorrection of a wavefront deviation of 100 nm, the correcting member 104should have a thickness d, which is approximately equal to 27 microns.However, placing and holding such a very thin element alone in theprojection lens is very difficult. In consideration of it, in thisembodiment, MgF₂ material of a thickness d is provided on a transportsubstrate, such as illustrated in FIG. 32B. More specifically, a MgF₂layer may be formed by vapor deposition and with a thickness d, upon asubstrate of fused silica or CaF₂ having no double refraction property,by which a desired structure can be accomplished. When the influence ofbirefringence is disregarded, the influence of a change in thickness d,of an order of a micron, to the aberration of the optical system can bedisregarded. Therefore, the thickness d of the correcting member 104 canbe determined on the basis of the magnitude of birefringence actuallyproduced in the projection optical system.

[0157] The birefringence correction amount can be adjusted not byadjusting the thickness d of the correcting member 104 as described, butby changing, through any dynamic means, the difference between theordinary ray refractive index n_(o)=1.427670 and an extraordinary rayrefractive index n_(e)=1.441134, which the MgF₂ crystal normallypossesses. FIG. 33 shows such an example wherein stress adjusting means130 is provided around a correcting member 104.

[0158] While MgF₂ is a positive uniaxial crystal, the difference inrefractive index (i.e., n_(o)−n_(e)) can be reduced by applying forcesuniformly and inwardly from the outside periphery of a circular flatplate. According to this method, the birefringence correction capacitycan be made variable, by changing the magnitude of the refractive indexdifference “n_(o)−n_(e)” while holding the plate thickness d constant.The stress adjusting means 130 comprises a metal belt, which is fixed tothe periphery of the correcting member 104. With the aid of a screw 131,it functions to uniformly and inwardly apply focus to the peripheralportion of the correcting member 104.

[0159] In accordance with the principle and arrangement described above,to a glass material such as CaF₂ or fused silica, which normally has nobirefringence characteristic, a double refraction property equivalent toa uniaxial crystal can be imparted. Thus, the correction capacity tobirefringence as produced by any other lens element in a projection lenscan be made variable continuously in a wide range.

[0160] The embodiment described above has been explained with referenceto an example wherein the lens elements constituting the projection lenshave an advancement phase axis distribution, extending radially in theoptical axis direction. However, in a case wherein the advancement phaseaxis is distributed concentrically with respect to the optical axis, thesign of birefringence to be corrected is inverted, such that the use ofa negative uniaxial crystal for the correcting member 104 is necessary.However, practically, there is no negative uniaxial crystal, which has ahigh transmission factor with respect to the ultraviolet region andwhich satisfies the conditions such as physical strength, for example.For this reason, the method having been described with reference to FIG.33 may be used in combination with a glass material such as fused silicaor CaF₂, for example, which normally shows no birefringence property. Byapplying uniform forces inwardly to the peripheral portion of a flatplate, the function equivalent to that of a negative uniaxial crystalcan be provided.

[0161] Further, although the correcting member 104 has been explainedabove as being a parallel flat plate, for correction of very finebirefringence not cancelled by adjustment of refractive index solely,the surface of the correcting member may be formed into a sphericalshape or an aspherical shape.

[0162]FIG. 34 is a schematic view of a main portion of a fifthembodiment of the present invention. This embodiment is a modified andimproved form of the fourth embodiment shown in FIG. 28. This embodimentdiffers from the fourth embodiment of FIG. 28 in that, in addition tothe correcting member 104 disposed between the image plane side finalface of the lens and the wafer surface, there is a second birefringencecorrecting member 142 adjacent to a stop 141.

[0163] The preceding fourth embodiment has been explained with referenceto an example wherein the object point and the image point are definedon the optical axis of the projection lens PL. In many cases, aprojection exposure apparatus uses a projection optical system, which istelecentric on its image side. In such a projection optical system, asshown in FIG. 35, chief rays (principal rays) 150 and 151 at differentimage heights are incident on a parallel flat plate perpendicularly,regardless of the image height. Also, the expansion of light rays aroundthe chief ray does not change with the image height. Therefore,basically, in accordance with the method described with reference to thepreceding embodiment, a birefringence property with respect to anyoff-axis object point can also be corrected like that for the on-axisobject point.

[0164] In FIG. 34, however, the influence of birefringence to threelight rays 144, 145 and 146 emitted from an object point P1, outside theoptical axis 143, and directed to an image point P2, differs from theinfluence in relation to an on-axis object point, as follows. First, asregards the chief ray 145, since it does not always go along the opticalaxis 143 of the projection lens P1 when there is not birefringence,there occurs a wavefront deviation in dependence upon the difference inpolarization direction. Second, as regards the chief ray 144, since itpasses through a peripheral portion of the lens where the magnitude ofbirefringence is relatively large as compared with that for the lightray 146, the influence of birefringence becomes large as a consequence.Thus, if only the influences of the lens elements 101, 102 and 103 areconsidered, a wavefront deviation φ (θ), which appears between thepolarization components 144P, 145P, and 146P parallel to the sheet ofthe drawing and the polarization components 144S, 145S and 146Sperpendicular to the sheet of the drawing, will be such as illustratedin FIG. 36. As regards the birefringence property that can be correctedby the correcting member 104, with respect to a chief ray of θ=0,basically, it is limited to one having a magnitude zero and having adistribution of a laterally symmetric shape. Therefore, sufficientcorrection will not be attainable, with respect to the distribution asshown in FIG. 36.

[0165] In the fifth embodiment, in consideration of it, the secondbirefringence correcting member 142 is used to transform, with regard tothe light from the off-axis object point P1, the distribution of thewavefront deviation φ (θ) as shown in FIG. 36 into a distribution φ ′(θ)such as shown in FIG. 37. Here, the second birefringence correctingmember 142 is made of a uniaxial optical crystal being disposed so that,like the correcting member 104, the crystal axis thereof (in thisexample, it corresponds to a principal axis or an optic axis) isoriented along the optical axis direction. Alternatively, it may be madeof a glass material having a property equivalent to the uniaxialcrystal. Since the three light rays 144, 145 and 146 are incident on thecorrecting member 142 substantially at the same angle, it provides aneffect that the distribution of wavefront deviation φ (θ) shown in FIG.36 is uniformly lowered by an amount corresponding to the deviationamount Δ. Here, the magnitude of the deviation amount Δ can be adjustedin accordance with the thickness of the correcting member 142 or themagnitude of the birefringence, which the correcting member 142possesses, this being exactly the same as has been described withreference to the correcting member 104 of the fourth embodiment.

[0166] The magnitude of birefringence to be corrected by the correctingmember 142 is practically variable with a distance h of the object pointP1 from the optical axis 143. If the value of distance h changes, itcauses a change in incidence angle of light emitted from the objectpoint P1 and incident on the correcting member 142. As a result, thebirefringence correction amount of the correcting member 142 changesaccordingly. Thus, the optical system can be arranged to ensure optimumcorrection with respect to every object height. Since the light emittedfrom an object point on the optical axis 143 is incident on thecorrecting member 142 substantially perpendicularly, the birefringencecorrection amount of the correcting member 142 for that light becomessubstantially equal to zero. Thus, in this case, the influence ofbirefringence is corrected by the correcting member 104 only.

[0167] By use of the correcting member 104 (first correcting member) andthe correcting member 142 (second correcting member) in combination asdescribed above, the influence of birefringence produced in relation toan off-axis object point can also be corrected very precisely. It shouldbe noted, however, that, with regard to the off-axis object point, asillustrated in FIG. 37, the asymmetry of the distribution around thechief ray cannot be completely corrected even by use of the correctingmember 142.

[0168] In consideration of it, the correcting member 142 may be usedparticularly to such a portion where the light rays 144, 145 and 146 areconverged as best seen in FIG. 38. If the angles with which the lightrays 144, 145 and 146 are incident on the correcting member 142 aredenoted by α1, α2 and α3, respectively, there is a relation α1>α2 α3.Thus, the system can be designed so that, with regard to thebirefringence correction amount, it becomes largest with respect to thelight ray 144 while it becomes smallest with respect to the light ray146.

[0169] As a result of the above, the distribution φ (θ), such as shownin FIG. 36, can be transformed into a distribution substantiallycompletely symmetrical about the chief ray, such as a distribution φ (θ)shown in FIG. 39. Thus, in combination with the correcting member 104,high precision correction can be accomplished even with regard to anoff-axis object point, like for an on-axis object point. It should benoted that two or more correcting members may be used to providebirefringence correction, respectively.

[0170]FIG. 40 is a schematic view of a main portion of a sixthembodiment wherein a projection optical system according to the presentinvention is incorporated into a stepper. Denoted in the drawing at 160is a reticle having a circuit pattern formed thereon, and denoted at 161is a projection optical system according to the present invention.Denoted at 162 is a wafer onto which the circuit pattern of the reticleis to be transferred. Illumination light 163 from an illumination system167 illuminates an illumination region 164 defined on the reticle 160,such that the circuit pattern formed in that region 164 is transferredin a reduced scale by the projection optical system 161 onto an exposureregion 165 of the wafer 162. In a stepper, after a pattern of thereticle 160 is simultaneously transferred to the wafer 162 in reductionscale, the wafer is moved stepwise through a predetermined distance and,subsequently, the exposure process is performed. This operation is maderepeatedly. The projection optical system 161 includes a birefringencecorrecting member of the present invention, such that the influence ofbirefringence of a glass material can be corrected and high precisionimaging performance is assured.

[0171]FIG. 41 is a schematic view of a main portion of a seventhembodiment wherein a projection optical system according to the presentinvention is incorporated into a step-and-scan type exposure apparatus.Denoted in the drawing at 170 is a reticle having a circuit patternformed thereon. Denoted at 171 is a projection optical system, anddenoted at 172 is a wafer onto which the circuit pattern of the reticleis to be transferred. Illumination light 173 from an illumination system167 illuminates an illumination region 174 defined on the reticle 170,such that the circuit pattern formed in that region 174 is transferredin a reduced scale by the projection optical system 171 onto an exposureregion 175 of the wafer 172. A step-and-scan type exposure apparatusdiffers from convention steppers, in the following points.

[0172] In a stepper, the pattern of a reticle 170 is simultaneouslytransferred to a wafer 172 in a reduced scale. In a step-and-scan typeexposure apparatus, as compared therewith, the circuit pattern of thereticle is illuminated with an illumination region 174 of a slit-likeshape while, on the other hand, the reticle 170 and the wafer 172 arescanned in synchronism with each other, by which the whole circuitpattern of the reticle 170 is transferred to the wafer in a reducedscale. The projection optical system 171 includes a birefringencecorrecting member of the present invention, such that the influence ofbirefringence of a glass material can be corrected and high precisionimaging performance is assured.

[0173] Semiconductor devices such as semiconductor chips (e.g., ICs orLSIs), liquid crystal panels or CCDs, for example, can be manufacturedby using a projection exposure apparatus as has been described withreference to any one of FIGS. 28-41. The procedure may include anexposure process for printing a device pattern of a reticle on a wafer,and a process for developing the exposed wafer, such as shown in FIGS.22 and 23.

[0174] In summary, in accordance with the embodiments of the presentinvention, as have been described with reference to FIGS. 28-41, thefollowing advantageous results are attainable.

[0175] (1) A projection optical system having a plurality of lenselements and including at least one birefringence correcting member,which is made of a uniaxial crystal having a principal axis in anoptical axis direction of the projection optical system, and/or of amaterial having a distortion distribution equivalent to the uniaxialcrystal, wherein the property of the at least one birefringencecorrecting member is determined so as to cancel birefringence to beproduced in relation to at least one of the lens elements.

[0176] (2) A projection optical system having a plurality of lenselements and including at least one birefringence correcting member,wherein the property of the at least one birefringence correcting memberis determined so as to cancel birefringence to be produced in relationto at least one of the lens elements.

[0177] (3) A projection optical system having a plurality of lenselements and including a variable-birefringence member.

[0178] While the invention has been described with reference to thestructures disclosed herein, it is not confined to the details set forthand this application is intended to cover such modifications or changesas may come within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is: 1-10. (Cancelled)
 11. A projection optical systemcomprising: a plurality of lenses that cause birefringence; and at leastone optical element for substantially eliminating the birefringencecaused by said plurality of lenses; wherein said at least one opticalelement is detachably mounted on said projection optical system.
 12. Aprojection exposure apparatus comprising: an illumination system forilluminating a reticle with light; and a projection optical system forprojecting a pattern of the reticle onto a wafer, said projectionoptical system including (i) a plurality of lenses that causebirefringence, and (ii) at least one optical element for substantiallyeliminating the birefringence caused by said plurality of lenses,wherein said at least one optical element is detachably mounted on saidprojection optical system.
 13. A device manufacturing method, comprisingthe steps of: exposing a wafer with a device pattern by use of aprojection exposure apparatus as recited in claim 12; and developing theexposed wafer.